Step crystallization

The less soluble diastereomer (S,S)-57 was obtained in >96% diastereomeric purity by six step-by-step crystallizations of this mixture from hexane or by three crystallizations, when a sample of (S,S)-57 was used as a crystallization seed (Scheme 20) <2003MC121, B-2003MI97>. 

A comparison of the steps crystallization and sweating shows, that the purification effect by sweating can be as high as the purification by crystallization (see also ( )). The loss of mass by sweating (7%-10%) is even less than the loss of mass by crystallization (20%-30%) (2) as to be found in industrial applications. 

This shows the possibilities to save energy and time by a good introduction of the sweating procedure and the co-ordination of the two steps crystallization and sweating. 

Because interface reaction and mass/heat transfer are sequential steps, crystal growth rate is controlled hy the slowest step of interface reactions and mass/heat transfer. For a crystal growing from its own melt, the growth rate may be controlled either by interface reaction or heat transfer because mass transfer is not necessary. For a crystal growing from a melt or an aqueous solution of different composition, the growth rate may be controlled either by interface reaction or mass transfer because heat transfer is much more rapid than mass transfer. Different controls lead to different consequences, including the following cases ... 

It is much easier to investigate the effect of surface irregularities on ordering using stepped crystal surfaces. Unlike in the case of uncontrolled surface defects on a (111) face of an fee metal for example, steps are readily detectable by LEED or FIM. They are likely to be ordered with a well-defined periodicity and surfaces can be prepared in such a way that steps are the predominant highest concentration surface defects. 

The chemisorption of over 25 hydrocarbons has been studied by LEED on four different stepped-crystal faces of platinum (5), the Pt(S)-[9(l 11) x (100)], Pt(S)-[6(l 11) x (100)], Pt(S)-[7(lll) x (310)], and Pt(S)-[4(l 11 x (100)] structures. These surface structures are shown in Fig. 7. The chemisorption of hydrocarbons produces carbonaceous deposits with characteristics that depend on the substrate structure, the type of hydrocarbon chemisorbed, the rate of adsorption, and the surface temperature. Thus, in contrast with the chemisorption behavior on low Miller index surfaces, breaking of C-H and C-C bonds can readily take place at stepped surfaces of platinum even at 300 K and at low adsorbate pressures (10 9-10-6 Torr). Hydrocarbons on the [9(100) x (100)] and [6(111) x (100)] crystal faces form mostly ordered, partially dehydrogenated carbonaceous deposits, while disordered carbonaceous layers are formed on the [7(111) x (310)] surface, which has a high concentration of kinks in the steps. The distinctly different chemisorption characteristics of these stepped-platinum surfaces can be explained by... 

In addition, the same studies that were carried out on the Pt(lll) crystal face result in reaction rates identical to those found on stepped crystal surfaces of platinum. These observations support the contention that well-defined crystal surfaces can be excellent models for polycrystalline supported metal catalysts. It also tends to verify Boudart s hypothesis that cyclopropane hydrogenolysis is an example of a structure-insensitive reaction. The initial specific reaction rates, which were reproducible.within 10%, are within a factor of two identical to published values for this reaction on highly dispersed platinum catalysts. The activation energies that were observed for this reaction, in addition to the turnover number, are similar enough on the various platinum surfaces so that we may call the agreement excellent. 

Crystallization processes comprise two sequential steps crystal nucleation followed by crystal growth. Kondepudi et al. demonstrated in a series of experiments that spontaneous symmetry breaking may be induced by growing crystals of non-chiral molecules such as sodium chlorate, binaphthyl, and p, p -dimethyl-chalcone, which crystallize as enantiomorphous crystals of... 

Lonza S.p.A. Furmaric acid Maleic anhydride Liquid-phase isomerization, two-step crystallization process 6 1991... 

Bear in mind the seven-step crystallization procedure ... 

In addition to the reaction or extraction step, crystallization, milling, and blending unit operations must be subject to qualification and validation. 

Huang F, Zhang H, Banfield IF (in prep.) Two-step crystal-growth kinetics observed in hydrothermal coarsening of nanocrystalline ZnS. 

TABLE 2.5j. Surface Structures on High-Miller-Index (Stepped) Crystal Faces ... 

STRUCTURES ON HIGH-MILLER-INDEX (STEPPED) CRYSTAL FACES... 

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